Fix NRVO for Gro variable.
Previously, we only marked the GRO declaration as an NRVO variable
when its QualType and the function return's QualType matched exactly
(using operator==). However, this was incorrect for two reasons:
1. We were marking non-class types, such as ints, as being NRVO variables.
2. We failed to handle cases where the canonical types were the same, but the actual `QualType` objects were different. For example, if one was represented by a typedef. (Example: https://godbolt.org/g/3UFgsL)
This patch fixes these bugs by marking the Gro variable as supporting NRVO only
when `Sema::isCopyElisionCandidate` is true.